struct ListNode* mergeTwoLists(struct ListNode* list1, struct ListNode* list2) {
    if (list1 == list2)
        return NULL;
    if (list1 == NULL)
        return list2;
    if (list2 == NULL)
        return list1;
    struct ListNode* head = NULL;
    struct ListNode* tail = NULL;
    while (list1 && list2)
    {
        if (list1->val < list2->val)
        {
            if (head == NULL)
            {
                head = tail = list1;
                list1 = list1->next;
            }
            else
            {
                tail->next = list1;
                tail = list1;
                list1 = list1->next;
            }
        }
        else
        {
            if (head == NULL)
            {
                head = tail = list2;
                list2 = list2->next;
            }
            else
            {
                tail->next = list2;
                tail = list2;
                list2 = list2->next;
            }
        }
    }
    while (list1)
    {
        tail->next = list1;
        tail = list1;
        list1 = list1->next;
    }
    while (list2)
    {
        tail->next = list2;
        tail = list2;
        list2 = list2->next;
    }
    return head;
}